In U.S. Pat. Nos. 4,052,648 (Nola) and 4,266,177 (Nola), there are disclosed power factor control systems which are particularly useful in connection with AC induction motors and which sample the line voltage and current through the motor and control the power input to the motor in proportion to the detected phase difference between the sampled voltage and current, whereby less power is provided to the motor in response to decreased loading of the motor. The control of input power is preferably effected by controlling the time during which a control triac connected in series with the motor is turned on.
A troublesome failure mode associated with triacs is that which causes the triac to effectively short circuit in one direction only. In effect, this failure mode is equivalent to placing a diode across the triac such that the triac passes uncontrolled current in one direction but still can be controlled in the opposite direction. If this failure mode occurs in phase controlled induction motor systems such as in the power factor controller described above, the unbalanced waveform results in a d.c. current component which can produce overheating and damage to the motor.
A further failure mode for triacs that presents particular detection problems is where the triac is effectively open circuited in one direction only. As explained below the present invention is concerned with detecting if a triac has failed shorted, or failed open, in one direction only and with providing the appropriate response thereto. It will be understood that while the invention is of particular value when used in connection with power factor controllers such as described above, the invention is not limited to such a use.